The isolated community of Fortitude in the Arctic Circle has never been the scene of a violent crime, so news of a murder rocks the residents. Sheriff Dan Anderssen investigates alongside newly arrived British detective Eugene Morton, while the killing is a disaster for Governor Hildur Odegard's plans to market Fortitude as an upmarket tourist destination.
Frank takes the law into his own hands in a bid to clear Liam's name, while Morton realises Henry may have vital information about Pettigrew's death. A polar bear carcass arrives at the research centre's lab and as Vincent and Natalie analyse the corpse, they develop a theory about the recent events in Fortitude, which they feel compelled to share with Hildur and Dan
Dan races into the frozen wilderness to locate Henry and Morton, but he is forced to reveal the truth surrounding the death of scientist Billy Pettigrew. Back in Fortitude, Natalie has a disturbing encounter with Jason, whose behaviour is becoming increasingly bizarre, while her and Vincent's investigation leads to a particularly strange development in the inquiries into recent events.
Vincent's investigation leads him into a life-threatening situation, while Markus offers an alternative theory on what is plaguing Fortitude. Dan and Hildur launch a desperate hunt for Jason as the need to locate the mammoth carcass escalates and Eric and Yuri go head to head out in the wilderness. With events taking an unexpected turn in the Arctic community, Sheriff Anderssen must make a devastating decision.